1. Field of the Invention
The present invention generally relates to a draft tube-type mixer including a propeller which pumps relatively large volumes of sludges or slurries and is positioned generally within or without a tank. In the case of anaerobic digestion of sewage sludges, these mixers act as low head, high volume sludge pumps and utilize a motor to drive a vertically-oriented sludge mixing propeller.
2. Description of the Prior Art
Draft tube mixers per se have been constructed and used for various applications, including high rate digestion processes, for many years. These devices include a draft tube(s) positioned within a digestion or other tank, which has a vertically mounted marine-type propeller or other paddle-like mixing vanes rotable with a vertical shaft driven by a motor. The motor and propeller are normally uni-directional in operation i.e. pumping either upwardly or downwardly. Prior art motors are sometimes reversible in order to try to "clear" or flush-off a propeller which has become fouled from rags, string or other debris or to put a marine vehicle in reverse. In reverse there is normally a large sacrifice in efficiency. The propellers used in most cases were designed for clean water use and have relatively short hub lengths. The construction is such as to cause unwanted materials (e.g. string and rags) which enter onto the inner 85-90% of the blade height to move toward the propeller hub. This results in those materials wrapping around the hub in a large ball. This increases the blade leading edge causing a reduction in pumpage and an increase in power draw. When the propeller is reversed it has been found that not much of the unwanted materials actually flushes off. Various marine-type propellers have two or more lobes or blades which have a root tracing a spiral or helical generatrix along a horizontal shaft hub. These are seen in U.S. Pat. Nos. 543,909 with uniform width blades; 1,015,540 with bifurcated ends; 1,307,106 with ends only attached to a shaft; 1,498,783 with a ichthyoidal hub profile; and 1,892,182 with concave/convex blade faces to direct water to the center of the propeller.
The problem of protecting pumps from rags and other unwanted material has also been solved to some degree by using recessed impellers. Such pumps however are not efficient in pumping the large volumes needed in draft tube mixers and do not perform satisfactory mixing.